Industrial plants having containers or tanks (“tanks”) generally need to regularly measure the level of liquid(s) or other product materials therein such as powders. There are several types of systems and techniques used for level measurement, which generally utilize time domain reflectometry (TDR) which relies on analyzing echoes.
For TDR-based measurements, there are contact level measurements, where a part of the system, such as a probe, contacts the material being measured, and non-contact level measurements where the level is measured with an antenna that is positioned above and without contacting the material to be measured. Guided wave radar (GWR) is a particular contact pulsed radar method used to measure the level of liquids or solids in a tank. Non-contact methods include ultrasound which uses high-frequency sonic (sound) waves to detect the level, and radar which uses electromagnetic energy at radio frequencies (RF) which propagates through free-space.
Radar-based tank gauging systems generally need an explosive proof and intrinsic safe (IS) system. IS limits the electrical and thermal energy to safe levels. Under normal conditions, the IS device is passive and allows the tank gauging system to function properly. Under fault conditions, the IS device protects the field circuit (e.g., radar level gauge (RLG)) by preventing excess voltage and current from reaching the hazardous area. The basic circuit for an IS barrier generally has three components that limit current and voltage: a resistor, at least two zener diodes, and a fuse. The resistor limits the current to a specific value known as the short circuit current, Isc. The zener diode limits the voltage to a value referred to as open circuit voltage, Voc. The fuse will blow when the zener diode conducts which interrupts the circuit that prevents the zener diode from burning up and allowing excess voltage to reach the hazardous area. There always are at least two zener diodes in parallel in each intrinsically safe barrier so that if one zener diode should fail, the other will operate providing complete IS protection.
An explosion proof arrangement includes a housing designed to contain an explosion. In a fault condition, the fault may create an explosion, but the container will contain the explosion. A radar level gauge can be installed in an explosion proof enclosure.
The RF signal in the non-contact radar must generally also have galvanic isolation for protection. Galvanic isolation is used where two or more electric circuits must communicate, but their grounds may be at different potentials. Galvanic isolation isolates functional sections of electrical systems to prevent direct current flow by breaking the direct current path. Energy or information is instead exchanged between the galvanically isolated sections by non-direct connections, such as capacitive, inductive (transformers), electromagnetic waves, optical, acoustical or mechanical arrangements. In tank gauging galvanic isolation is used for safety by preventing explosions in the tank, particularly when there is a flammable material in the tank.